Delay cartridges that employ pyrotechnic delays are used to sequence and operate aircrew escape systems. The capability of a cartridge to deliver energy at a programmed delay time after an initiation stimulus determines whether or not a pilot shall successfully eject from an aircraft in an emergency. A given delay cartridge lot qualified at -65.degree. F. and +200.degree. F. temperature will have a lower and upper 3.sigma. limit on delay time at each temperature. The time span between the lower 3.sigma. limit at +200.degree. F. and the upper 3.sigma. limit at -65.degree. F. comprises a statistical delay time envelope in which the delay times of cartridges from the qualified lot will fall.
Conventional delay cartridges generally include: case; percussion primer; flash chamber; a delay body, plug, or housing, surrounding the pyrotechnic delay column of suitable composition and an igniter composition for igniting the delay column; a transfer charge; a propellant output charge; and closure disc. During burning, heat is continually being transferred to the delay body from the delay column since the delay column reaction is highly exothermic while the delay body is at a much lower temperature, thus acting as a heat sink. In most situations, this heat transfer to the delay body slows the burning rate of the delay column and alters the predictability of the time delay. Although attempt to utilize delay bodies fabricated from low thermal conductivity materials have been made in the past, marginal improvement in the statistical delay time envelope between the extreme temperatures have been achieved. Moreover, materials with low thermal conductivities are difficult to utilize as delay bodies.
Most pyrotechnic delay compositions are less temperature dependent at higher ambient temperatures. In other words, a change in temperature when the ambient temperature is high will produce a smaller change in delay burn time than a temperature change occurring at a lower ambient temperature. Therefore, performance of the delay could be enhanced by providing an elevated-temperature environment during delay column reaction. The teachings of U.S. Pat. No. 3,851,586 issued Dec. 3, 1974 show how to minimize or eliminate ambient temperature effects upon the burning rate and variability of the delay. Here, the pyrotechnic delay column is surrounded with a volume of material that is easily ignited, high-heat producing, and fast burning relative to the burning rate of the delay column. Heat released by this material heats the body of the delay system, creating an elevated temperature environment for the delay column which is desirable to enhance the predictability of the delay variable and to minimize adverse effects of the actual ambient temperature. However, the use of pyrotechnic heating in a delay cartridge to condition the delay column to higher temperature results in increased pressure and higher temperature in the flash chamber, the enclosed chamber in a delay cartridge that is formed in the internal cylindrical spaces of the cartridge casing between the percussion primer and the delay plug with its delay column. The delay time of the delay cartridge is strongly dependent upon the temperature and pressure in the flash chamber. These parameters affect burn rate directly and through diffusion of combustion products from the flash chamber into the unburned delay column. The volume of the flash chamber is most important, because it is reflected in flash chamber pressure and temperature. Unfortunately, the resulting increased pressure and temperature in the flash chamber with ambient temperature from pyrotechnic heating degrades the desired temperature compensation benefits.
Special formulations of the delay ingredients, especially with regard to heavy metal soaps, are provided by the teachings of U.S. Pat. No. 3,726,730 issued Apr. 10, 1973. Although these formulations have met with limited success, formulation of these delays to the exact specifications that produce the desirable properties is hard to duplicate.
The apparatus and methods disclosed in this application extend the delay time at elevated temperatures, since a delay column normally takes less time to burn at high temperatures than at low temperatures. Thereby the statistical delay time envelope is reduced and the overall delay accuracy of the cartridge is increased.